Do not specity product variables. Specifying distribution types, minimum lumens per watts, and maximum drive currents (power condition) can prevent you from discovering the product that may offer the best value for your needs. Defining fixed variables such as pole height, pole spacing, and roadway dimensions are necessary, but it’s important not to constrain the process in other areas.
Specify product standards, not features. Features imply some level of performance; standards ensure a level of performance has been met. Using a standard better ensures some level of performance and reduces risk.
Applications and products are unique
Spec sheets can offer a wealth of information, but they can’t tell you how well a particular product will perform optically on a specific roadway. Two products that offer the same light distribution classification and produce the same amount of initial light output can perform very differently at the application level.
Consider target efficacy. Efficacy, or lumens per watt (LPW), is sometimes used to evaluate the efficiency of an LED luminaire. It defines how much light per watt of energy a luminaire delivers, but does not consider where the light goes. Keeping the light on target or target efficacy (LPW on target) is what’s critical. Lighting which is off-target offers no value and in some instances can create challenges, such as when undesirable light falls off-target and shines into residents’ homes.
Understand lumen depreciation. All light sources produce less light over time, but LEDs depreciate (lose light) much more slowly than traditional sources. Thus, re-lamp cycles can be eliminated and streetlights can be designed to be near maintenance-free for more than a decade. However, LEDs depreciate at different rates depending on LED chip package, drive current (power condition) to which the LED chip package is exposed, and the product’s thermal management design.
It is important to request specific and credible lumen maintenance numbers from manufacturers for the product and application time frame being considered. This number should also vary based on the ambient temperature of the installation itself. Without credible, manufacturer-specific data, there’s no way to predict performance or do lighting design.
Look to success stories. Talk to other communities that have already installed LED streetlights. Learn from their experiences. Begin small, but start now and gain knowledge and experience that can pay big dividends today and down the road.
—Duane Henderson (firstname.lastname@example.org) is senior product manager for Cree Inc., Durham, N.C. Visit www.cree.com.